The present invention is directed to a positioning device for an optical data storage. The device includes an optical system for generating a polarized, parallel laser light beam and for out coupling of a reflected light beam, an arrangement for evaluating the reflected light beam and a sensing head arranged radially movable relative to the storage disk for track selection with the effective vertical distance of the sensing head from the surface of the storage disk being variable for readjustment of the focussing of the beam.
The properties of the respective positioning devices have always played a significant part in disk storage whether they are for magnetic disk storage or optical disk storage. Technological advances have enabled an increase in the storage capacity with the increase in the track density being of particular interest but, at the same time, all possible efforts were also undertaken to shorten the mean access time in order to increase the performance capability of the respective disk storage.
Therefore, there has been no lack of attempts to satisfy the demands for short access time and a high tracking density which are contradictory per se. In a positioning system, the moved mass will always normally occur and this includes the read unit moved essentially radially relative to the storage disk. Particularly in a magnetic disk storage, advances in technology, particularly in the magnetic heads, have succeeded in substantially reducing the mass which must necessarily be moved and thus, is likewise creating faster positioning means. In optical storages, the reduction of mass is far more difficult because of the optical imaging system.
As known, relative high access speeds, for example a fast radially movement of the positioning means given track changes, can also be more easily achieved with a positioning device which is executed in multi-stages. A rough positioning of the head unit thereby occurs first whether it is a magnetic write/read head or an optical sensor an.d this rough positioning attempts to position the head to the track region as closest as possible to the data track to be selected or to this track itself. This is followed by fine positioning, which sets the head unit to the selected track and/or.holds the track position. This division of the positioning events into two steps having a high translational speed in the first phase and a precise fine adjustment in the second phase allows an access time which is optimized on average with the given boundary conditions upon employment of the two control systems.
Optical data storages are currently still frequently employed only for archiving purposes so that the mean access time given continual writing and/or reading of great data quantities still does not play a decisive part. However, if optical data storages are to compete with magnetic storage devices, for example if optical data storages are to be employable for operating modes having frequent track changes, then extremely short access times must also be provided in this type of storage.
In this context, U.S. Pat. No. 4,545,046, whose disclosure is incorporated by reference thereto and which is based on the same Dutch Patent Application as European patent application No. 0,114,082, discloses a positioning device for an optical data storage which is fashioned as a linear positioner. For example, a sensing unit is displaceably arranged on a pair of guide rods. The drive for the storage disk spindle is also provided at an end of the guide rods at the same time. In the case of an optical system for generating a read beam and for evaluating a reflected laser beam is stationarily arranged and this is also positioned at the other end of the guide rods.
Whereas the spatial alignment of modules decoupled from one another is in the foreground of the known device, it also teaches in a direction which leads away from what are referred to as integrated sensors, for example, sensors, which combine all the optical and electro-optical elements. However, the mass of the movable sensing unit is still extremely high.
Another principle for resolving this problem would be to limit the absolute path length of the read head in the track selection. An example of a trial device of this direction is known from R. L. Garwin "Optics for Beam Addressable Files", IBM Technical Disclosure Bulletin, Vol. 15, No. 2, July 1972, pp. 494-495. In this device, instead of a single imaging lens, a plurality of lenses are arranged next to one another and are combined in a read unit. A respective group of data tracks which, however, do not lie immediately next to one another are thus, selectable. A single data track from this group is selected via each of these lenses with differently deflected laser beams. A data track, which lies in between these single tracks, must thus be selected by mechanical radial motion of the sensing unit.
A special disadvantage of this known solution is the proliferation of the optical imaging elements including a large reflector surface in the beam path. Thus, a great mass must always be mechanically moved for spindle adjustment and track follow-up. This property or characteristic obviously limits the possible track density.